[0001] The present invention relates to a method for curing organosiloxane compositions
to form a silicone rubber. More specifically, the present invention relates to a novel
method for curing organosiloxane compositions which use an organohydrogenpolysiloxane
as the curing agent in the absence of a curing catalyst or an organoperoxide.
[0002] Known methods for curing organosiloxane to form a silicone rubber include 1) the
condensation of silanol substituted organopolysiloxanes with organosilicon compounds
having a plurality of hydrolyzable groups, for example, alkoxysilanes, acetoxysilanes
or aminoxysilanes; 2) the free radical initiated polymerization of diorganopolysiloxanes
in the presence of organoperoxides; and 3) a hydrosilylation reaction between compounds
containing silicon-bonded hydrogen atoms and organopolysiloxanes containing ethylenically
unsaturated hydrocarbon radicals, for example, silicon-bonded vinyl radicals, in the
presence of a platinum catalyst.
[0003] The necessity for using platinum metal or a platinum compound to catalyze curing
of organpolysiloxanes by a hydrosilation reaction is taught in the literature, including
a text entitled "Chemistry and Technology of Silicones" by Walter Noll, published
as an English translation in 1968 by Academic Press, New York; and an article entitled
"Silicone Elastomer Developments 1967-1977" by E.L Warrick et al. [Rubber Chemistry
and Technology, 52 (3), 1979].
[0004] Recently disclosed methods for curing organosiloxane compositions include curing
by means of ultraviolet irradiation; curing by means of high-frequency radiation,
infrared radiation or electron beams and the polymerization of mercapto substituted
diorganopolysiloxanes in the presence of organoperoxides. Of these known methods,
curing in the presence of organoperoxides and curing via a platinum catalyzed hydrosilylation
reaction are the ones most generally used. Curing of organosiloxane compositions using
organoperoxides suffers from the problem of organoperoxide decomposition residues
remaining in the cured product, with the result that the product requires post-vulcanization
to remove or inactivate these residues.
[0005] The disadvantages associated with curing by a hydrosilation reaction are poor storage
stability and short use time of the curable organosiloxane composition.
[0006] To avoid these problems, the present inventors examined various methods for curing
organosiloxane compositions with the objective of providing a novel method for curing
these compositions without the need for a curing catalyst or an organoperoxide.
[0007] The present invention is based on the discovery that a cured silicone rubber can
be produced by heating under superatmospheric pressure an organosiloxane composition
comprising an alkenyl-containing organopolysiloxane, an organohydrogenpolysiloxane
and finely divided silica in the absence of a hydrosilation catalyst.
[0008] The present invention provides a method for curing an organosiloxane composition,
said method consisting essentially of heating said composition at a temperature of
at least 150°C and under a pressure of at least 9.8 kilopascals for a sufficient time
to cure said composition, where said composition comprises
(A) 100 parts by weight of an organopolysiloxane with the average unit formula RaSiO(4-a)/2 where R represent an unsubstituted or substituted monovalent hydrocarbon radical
where the substituent is at least one member selected from the group consisting of
halogen atoms and the cyano group, the average value of ª is from 0.8 to 2.2, and
said organopolysiloxane contains at least 2 lower alkenyl radicals in each molecule,
(B) an organohydrogenpolysiloxane having at least 2 silicon-bonded hydrogen atoms
in each molecule in an amount sufficient to provide at least 1 mole of silicon-bonded
hydrogen atoms per 1 mole of alkenyl radicals in ingredient (A), and
(C) 5 to 200 parts by weight of finely divided silica, and where said composition
does not contain a hydrosilation catalyst.
[0009] The characterizing features of the present method include the presence of finely
divided silica and the absence of a hydrosilation catalyst or organoperoxide for promoting
curing of the organosiloxane composition. The composition is cured by heating it under
superatmospheric pressure.
[0010] By way of explanation, with regard to the organopolysiloxane comprising ingredient
A, R in the aforementioned formula represents a monovalent hydrocarbon radical or
a substituted monovalent hydrocarbon radical wherein at least a portion of the carbon
atoms contain at least one halogen atom or cyano group as substituents. The R radicals
can be identical or different and can alkyl such as methyl, ethyl and propyl; alkenyl
such as vinyl and allyl; aryl such as phenyl and tolyl; cycloalkyl such as cyclohexyl;
or any of these radicals in which at least a portion of the hydrogen atoms have been
replaced by halogen or cyano, for example, chloromethyl, trifluoropropyl and cyanomethyl.
At least 2 of the R radicals in each molecule must be alkenyl.
[0011] The organopolysiloxane referred to herein as ingredient A can be linear or branched.
At least 50 mol% of the R radicals are preferably methyl. The terminal groups of ingredient
A are not specifically restricted, and can be silanol groups, alkoxy groups and triorganosiloxy
groups such as trimetnylsiloxy, dimethylphenylsiloxy, dimethylvinylsiloxy and/or methylphenylvinylsiloxy.
The average value of the number represented by ª is from 0.8 to 2.2 and preferably
from 1.95 to 2.05.
[0012] The organohydrogenpolysiloxane referred to herein as ingredient B of the present
compositions is the crosslinker for curing the composition. The molecular configuration
of this ingredient can be straight chain, cyclic or branched chain. This ingredient
must contain at least 2, preferably at least 3, silicon bonded hydrogen atoms in each
molecule. The degree of polymerization (DP) of this ingredient must be at least 2
and is preferably 3 or greater.
[0013] The molar ratio of silicon-bonded hydrogen atoms in ingredient B relative to alkenyl
radicals in ingredient A is preferably in the range of 5:1 to 50:1 in order to achieve
a satisfactory cure. The amount of ingredient B is generally from 1 to 10 parts by
weight per 100 parts by weight of ingredient A.
[0014] The finely divided silica, referred to herein as ingredient C, is essential to achieve
crosslinking of ingredient A by reaction with ingredient B in the absence of a platinum-containing
catalyst. In the absence of this finely divided silica, the curing reaction does not
occur under the conditions of the present method.
[0015] Ingredient C is exemplified by precipitated silica, which can be produced by wet
methods, and fumed silica, which can be produced by dry methods. Ingredient C is present
within the range of from 5 to 200 parts by weight, preferably from 10 to 100 parts
by weight, per 100 parts by weight of ingredient A. Failure to observe this range
is undesirable because curing will be unsatisfactory at silica concentrations below
5 parts by weight, while the workability of the composition will be reduced at silica
concentrations of above 200 parts by weight.
[0016] In accordance with the present method, a mixture comprising ingredients A, B, and
C is heated at a temperature of at least 150°C under superatmospheric pressure. This
pressure is preferably at least 9.8 kilopascals (kPa), and most preferably at least
98 kPa. It appears that the curing reaction will not proceed at pressures below 9.8
kPa.
[0017] Curing of the present composition will occur at 150°C, however a temperature of at
least 180°C is preferred, with temperatures within the range of from 200 to 250°C
being most preferred.
[0018] In addition to the aforementioned ingredients A, B, and C, additional ingredients,
including finely divided quartz powder, diatomaceous earth, zinc white, aluminum silicate,
iron oxide, cerium oxide, titanium hydroxide, asbestos, glass fiber, pigments and
heat stabilizers can be blended into the present compositions as desired unless this
adversely affects the objectives of the present invention.
[0019] In accordance with the present invention, a composition comprising specified quantities
of lower alkenyl-containing organopolysiloxane, an organohydrogenpolysiloxane and
finely divided silica is cured by heating it at a temperature of at least 150°C under
a pressure of at least 9.8 kilopascals. The present method eliminates the requirement
for and disadvantages associated with a hydrosilation catalyst or an organoperoxide.
[0020] The following examples describe preferred embodiments of the present invention, and
should not be interpreted as limiting the scope of the invention as defined in the
accompanying claims. All parts and percentages in the examples and comparison examples
are by weight unless otherwise specified, and viscosity values are measured at 25°C.
EXAMPLE 1
[0021] An organosiloxane composition of this invention was prepared by mixing to homogeniety
100 parts of a dimethylvinylsiloxy-terminated organopolysiloxane gum containing an
average of 3000 repeating units per molecule, 0.5 weight % vinyl radicals, 99.8 mol%
dimethylsiloxane units, and 0.2 mol% methylvinylsiloxane units; 45 parts wet-process
precipitated silica (Nipsil LP from Nippon Silica Co., Ltd.) and 5 parts of a trimethylsiloxy-terminated
methylhydrogenpolysiloxane having a viscosity of 2x10⁻⁵m²/sec and a silicon bonded
hydrogen atom content of 1.0 wt%. This composition was placed in a metal mold and
then heated at 200°C under a pressure of 2.45 megapascals for 10 minutes. After cooling
the metal mold, the organosiloxane composition was demolded and found to be cured.
[0022] The physical properties of this cured silicone rubber product were measured in accordance
with Japanese Industrial Standards (JIS) test procedure K-6301. The product exhibited
a hardness (JIS A) of 54, a tensile strength of 8.6 megapascals and an elongation
at break of 480%.
[0023] For purposes of comparison, the same organosiloxane composition was placed in the
metal mold and heated at 200°C in the absence of superatmospheric pressure for 10
minutes, cooled and demolded. In this instance, the organosiloxane composition was
not cured.
EXAMPLE 2
[0024] 100 Parts of the organopolysiloxane gum described in the preceding Example 1 was
mixed to homogeneity with 50 parts dry-process silica (Aerosil 300 from Nippon Aerosil
Co., Ltd) and 5 parts of the methylhydrogenpolysiloxane described in the preceding
Example 1 to obtain an organosiloxane composition of this invention. This composition
was placed in a metal mold and heated at 200°C under a pressure of 2.45 megapascals
for 10 minutes. After the metal mold had cooled, the composition was demolded and
found to be a cured silicone rubber. The physical properties of this cured product
are measured in accordance with JIS K-6301. The rubber exhibited a hardness value
(JIS A) of 38, a tensile strength of 0.78 megapascals and an elongation at break of
600%.
[0025] For purposes of comparison, the same organosiloxane composition was placed in the
metal mold, heated at 200°C under atmospheric pressure for 10 minutes, cooled and
then demolded. In this instance, the organosiloxane composition was not cured.
EXAMPLE 3
[0026] An organosiloxane composition prepared as described in the preceding Example 1 was
placed in a metal mold and then heated at 160°C under a pressure of 9.8 megapascals
for 10 minutes. After cooling the metal mold, the organosiloxane composition was demolded
and found to be a cured silicone rubber. The hardness of this cured product was 50,
measured in accordance with test procedure JIS A.
1. A method for curing an organosiloxane composition, said method consisting essentially
of (1) heating said composition at a temperature of at least 150°C and under a pressure
of at least 9.8 kilopascals for a period of time sufficient to cure said composition,
where said composition comprises
(A) 100 parts by weight of an organopolysiloxane with the average formula RaSiO(4-a)/2 where R represents an unsubstituted or substituted monovalent hydrocarbon radical
where the substituent is at least one member selected from the group consisting of
halogen atoms and the cyano group, the average value of ª is from 0.8 to 2.2, and
said organopolysiloxane contains at least 2 lower alkenyl radicals in each molecule,
(B) an organohydrogenpolysiloxane having at least 2 silicon-bonded hydrogen atoms
in each molecule in an amount sufficient to provide at least 1 mole of silicon-bonded
hydrogen atoms per 1 mole of alkenyl radicals in ingredient (A), and
(C) 5 to 200 parts by weight of finely divided silica,
and where said composition does not contain a hydrosilation catalyst.
2. A method according to claim 1 where each R is individually selected from the group
consisting of methyl, propyl, vinyl, allyl, phenyl, tolyl, cyclohexyl, chloromethyl,
trifluoropropyl and cyanomethyl, and the degree of polymerization of the organohydrogenpolysiloxane
is at least 3.
3. A method according to claim 2 where at least 50% of the R radicals are methyl,
the organopolysiloxane is a diorganopolysiloxane, the terminal units of said diorganopolysiloxne
are dimethylvinylsiloxy units and the curing temperature of said composition is at
least 180 degrees C.
4. A method according to claim 3 where said diorganopolysiloxane is a dimethylsiloxane/methylvinylsiloxane
copolymer, said silica is a wet process precipitated silica and is present at a concentration
of from 10 to 100 parts by weight per 100 parts by weight of said copolymer, and the
composition is cured at a temperature of from 200 to 250 degrees C. under a pressure
of at least 98 kPa.